July 1, 2014

Scientists from the Universities of Sydney, Harvard, Stanford, and MIT have bio-printed artificial vascular networks mimicking the body’s circulatory system.

These networks are necessary for growing large complex transplantable tissues and organs for people affected by major diseases and trauma injuries.

“Thousands of people die each year due to a lack of organs for transplantation,” says study lead author and University of Sydney researcher Luiz Bertassoni. ”Many more are subjected to the surgical removal of tissues and organs due to cancer, or they’re involved in accidents with large fractures and injuries.

“Imagine being able to walk into a hospital and have a full organ printed — or bio-printed, as we call it — with all the cells, proteins and blood vessels in the right place, simply by pushing the ‘print’ button in your computer screen. We are still far away from that, but our finding is an important new step towards achieving these goals.

“At the moment, we are pretty much printing ‘prototypes’ that, as we improve, will eventually be used to change the way we treat patients worldwide.” The research challenge was networking cells with a blood supply. Cells need ready access to nutrients, oxygen and an effective “waste disposal” system to sustain life. That’s why “vascularization” — a functional transportation system — is central to the engineering of biological tissues and organs.

“One of the greatest challenges to the engineering of large tissues and organs is growing a network of blood vessels and capillaries,” says Bertassoni. “Cells die without an adequate blood supply because blood supplies oxygen that’s necessary for cells to grow and perform a range of functions in the body.”

“To illustrate the scale and complexity of the bio-engineering challenge we face, consider that every cell in the body is just a hair’s width from a supply of oxygenated blood. Replicating the complexity of these networks has been a stumbling block preventing tissue engineering from becoming a real world clinical application.”

The research achievement

Schematic representation of bioprinting of agarose (a polysaccharide obtained from agar) template fibers and subsequent formation of microchannels via template micromolding. A) A bioprinter equipped with a piston fitted inside a glass capillary aspirates (draws by suction) the agarose (inset). After gelation (freezing to convert to gel) at 4 degrees C, agarose fibers are bioprinted at predefined locations. B) A hydrogel precursor is casted over the bioprinted mold and photocrosslinked. C) The template is removed from the surrounding photocrosslinked gel. D) Fully perfusable microchannels are formed. (Credit: University of Sydney)

Using a bio-printer, the researchers fabricated a multitude of interconnected tiny fibers to serve as the mold for the artificial blood vessels. They then covered the 3D printed structure with a cell-rich protein-based material, which was solidified by applying light to it.

Lastly they removed the bio-printed fibers to leave behind a network of tiny channels coated with human endothelial cells, which self-organized to form stable blood capillaries in less than a week.

The study reveals that the bioprinted vascular networks promoted significantly better cell survival, differentiation, and proliferation compared to cells that received no nutrient supply.

According to Bertassoni, a major benefit of the new bio-printing technique is the ability to fabricate large three-dimensional micro-vascular channels capable of supporting life on the fly, with enough precision to match individual patients’ needs.

“While recreating little parts of tissues in the lab is something that we have already been able to do, the possibility of printing three-dimensional tissues with functional blood capillaries in the blink of an eye is a game changer,” he says.

“Of course, simplified regenerative materials have long been available, but true regeneration of complex and functional organs is what doctors really want and patients really need, and this is the objective of our work.

Abstract of Lab on a Chip paper

Vascularization remains a critical challenge in tissue engineering. The development of vascular networks within densely populated and metabolically functional tissues facilitate transport of nutrients and removal of waste products, thus preserving cellular viability over a long period of time. Despite tremendous progress in fabricating complex tissue constructs in the past few years, approaches for controlled vascularization within hydrogel based engineered tissue constructs have remained limited. Here, we report a three dimensional (3D) micromolding technique utilizing bioprinted agarose template fibers to fabricate microchannel networks with various architectural features within photocrosslinkable hydrogel constructs. Using the proposed approach, we were able to successfully embed functional and perfusable microchannels inside methacrylated gelatin (GelMA), star poly(ethylene glycol-co-lactide) acrylate (SPELA), poly(ethylene glycol) dimethacrylate (PEGDMA) and poly(ethylene glycol) diacrylate (PEGDA) hydrogels at different concentrations. In particular, GelMA hydrogels were used as a model to demonstrate the functionality of the fabricated vascular networks in improving mass transport, cellular viability and differentiation within the cell-laden tissue constructs. In addition, successful formation of endothelial monolayers within the fabricated channels was confirmed. Overall, our proposed strategy represents an effective technique for vascularization of hydrogel constructs with useful applications in tissue engineering and organs on a chip.

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There’s a company here in San Diego called Organovo that claimed to have already done this several years ago. I watched the CEO do a presentation at a local brewery in 2012 and the stuff he was showing was very exciting indeed, with pictures of completed vessels that they had printed in their lab. They were paying the bills in the interim by doing a sort of advanced version of ELISA, in which the matrix was a 3D liver tissue printed on the bottoms of wells of 96-well plates to see the effects of drugs on whole tissues, not just blood. Their ultimate goal however is to print whole organs, in addition to the blood vessels that they’ve already apparently had some success with.

While I applaud this kind of research, I still think that once again to achieve best results at lowest possible costs, one has rather to imitate Nature itself, i.e. here in the context of the essential process of proper vascularization development (angiogenesis) to support new organs generation.

Nature tells us that angiogenesis is induced via hypoxia (and the necessity for cell preservation to compensate for lack of oxygen). In that typical setup, the cell nucleus activates numerous target gene products having for end result to promote the secretion of growth factors (such as VEGF, FGF, and TGF) inducing themselves signaling pathways (including PLCγ, PI3K, Src, and Smad signaling) that result in endothelial cell proliferation, hence increased vascular permeability, and cell migration.

This is endogenous angiogenesis and not a priori exogen vascularization acting like a given, a priori, scaffold where the cells have to migrate like what is achieved in this study.

Once again, the emphasis has to be put on systems biology and what I would like to call mathematical models of general embryology, a discipline alas not yet formalized and still to be abstracted from repeated observations over the development of biological tissues and organs.

After thinking about it awhile, and reading the article again a little more carefully, it occurs to me that when we have 3-D printers that can print computer circuits by using a cluster of heads fed by different materials (some conductors, some semiconductors, some insulators) then we will also have bio-printers with a number of heads that can shoot out differing cells.

The cells will be cultured separately with all the factors that you are thinking off, melajara, so that the correct type of cell will be injected right where you want it.

The cells will arrive in just the place where they can do the job that their DNA is ready to put them to work at.

But wouldn’t you think that those in the field who work with this stuff daily would have considered your idea? Not trying to be difficult but I see this frequently and have wondered about the odds that experts in the field never considered our suggestions. Probably close to zero.

Saying that, I agree with a biological approach as a faster pathway. I am very slowly going blind and I’ve discussed mechanical fixes (he is far more pessimistic than I) but last time he said he is convinced that a biological approach would be more straight-forward and have a greater chance of success.

How would something like that work in vitro though, as they’re doing here? It sounds like what you’re describing would only be possible in vivo and I think we’re a long way off from being able to manipulate a fully differentiated living organism with that level of precision. It would seem that there’s a reason that most tissues can’t spontaneously regenerate on their own, and it may be equally as difficult to synthetically do so in vivo for those same reasons. If this is true, removing the process from the body and re-introducing it when the synthesis is complete may be the only way to go.